1. Field of the Invention
This invention relates to coating agents for leather and leather substitutes, which coating agents contain a polyurethane starting material. They may be used for coating and dressing leather and leather substitute materials from solution. By virtue of the special composition of the polyurethane, the dressings and coatings obtained have very desirable qualities which in their totality are novel and unexpected.
2. Description of the Prior Art
The coating applied to leather or synthetic leather, also known as final lacquer, dressing or finish, is intended to impart to the substrate a particular appearance (color shade or gloss) or a particular "hand" and to provide resistance to external influences.
The coating must therefore adhere firmly to the substrate, have great hardness and flexibility under bending and be resistant to wet and dry abrasion. When applied to synthetic leathers (e.g., PVC leather substitute) which contain plasticizers and/or dyes, the dressing must completely prevent migration of the plasticizers or dyes such as organic pigments to the surface since such migration would make the coated articles tacky on the surface and unsightly. If an article is composed of differently colored parts, the dressing must prevent the organic dyes from "bleeding". By this is meant the migration of organic dyes from colored to uncolored or differently colored portions of material by means of plasticizers or fats.
Another problem that has hitherto only been solved unsatisfactorily with the known polyurethane dressings is the question of solvents. Particularly when using polyurethanes based on aromatic diisocyanates it has been necessary to use solvents which are not ideal for dressings. Dimethylformamide, for example, which is frequently used, has the disadvantage that it partly dissolves the substrate or causes it to swell considerably and owing to its high boiling point it only slowly evaporates from the film of dressing at room temperature. Solutions which contain dimethylformamide may, therefore, severely attack the substrate or even destroy it.
Another commonly used polyurethane solvent is tetrahydrofuran but this also has disadvantages for use in the leather industry. Thus it has an unpleasant smell and very readily forms peroxides so that it is difficult to recover by distillation.
The coating agents known in the art also include copolyamides which are soluble in mixtures of hydrocarbons and alcohols. These products may also be used to produce dressings which have a pleasant hand and high resistance to diffusion but they have the serious disadvantage of being highly sensitive to alcohols, e.g. aqueous ethanol.
It is an object of the present invention to provide a one-component polyurethane system suitable for dressing or coating leather or leather substitutes, which does not have the disadvantages mentioned above and has improved qualities compared to the known polyurethane dressing agents. The dressing agent should in particular constitute an improved barrier to plasticizers and have great hardness and folding strength, impart a dry, pleasant hand to the substrate and be soluble in solvents such as aromatic hydrocarbons, ketones and alcohols and form solutions which are stable in storage.
This problem has been solved by using polyurethane ureas prepared from different macrodiols (as soft segments) which have a special structure.
It is known that the use of different macrodiols in polyurethanes improves the mechanical characteristics, in particular the tensile strength. Thus, for example, a process for the preparation of polyurethanes using different macrodiols has been described in German Auslegeschrift No. 2,623,961. This process, however, requires the use of separately prepared isocyanate prepolymers which are then mixed before the polyurethane is formed. This three-stage process, therefore, has the disadvantage of being complicated and laborious. Moreover, the products obtained are not ideally suitable for the dressing of leather and leather substitutes.